EPSILON AND (Epsilon Andromedae). To the eye alone, stars give few clues clue as to their natures. The only observables to the naked eye are apparent brightness (which depends on real luminosity and distance), and subtle color (reddish ones quite readily popping out). Actual motions relative to each other are insensible. Most of the stars of the nightly sky are in more or less circular orbits about the center of our Galaxy, moving relative to each other by only 10 to 20 kilometers per second. Not so Epsilon Andromedae, which shines in western Andromeda not far from the Great Square of Pegasus. While looking much like the stars around it, it is a local visitor from a distant neighborhood. This fourth magnitude (4.37) class G (G8) giant star, with a temperature of 4930 Kelvin, lies 169 light years away, from which we find a luminosity 52 times that of the Sun, a radius 9.9 times solar, and a rotation period less than twice the solar 25 day period. Direct measure of its angular diameter gives a radius of 9.8 solar, showing that all the parameters are right on the mark. From the luminosity and temperature, Epsilon And is a 2.4 solar mass "clump star," one that has settled in as a mid luminosity giant as it quietly fuses the helium in its core into a mixture of carbon and oxygen. Only 650 million years old, it began life as a class B9 hydrogen-fusing dwarf. Epsilon's claim to non-ordinary stature lies in its motion. Moving away from us at 84 kilometers per second, and across the line of sight at 83, its total velocity relative to the Sun is a very high 118 kilometers per second. The Sun orbits within our Galaxy at relatively constant distance of 28,000 light years from the center (with an orbital speed of about 220 kilometers per second). On a highly elliptical orbit, Epsilon And comes to us from way inside the Galaxy, at a minimum distance from the center of 14,000 light years, and is passing us, heading for the distant outer Galactic neighborhood 35,000 light years from the center. Typical of high-speed stars, it is also low in metals, its iron content only a quarter that of the Sun (the Sun actually being somewhat high for its own neighborhood).
Written by Jim Kaler. Return to STARS.